Employee benefits (Tables)

12 Months Ended

Dec. 31, 2015

Employee benefits

Change in benefit obligations, plan assets and funded status recognized in balance sheet

	2015			2014			2015			2014
($ in millions)	Defined pension benefits						Other postretirement benefits
Benefit obligations at January 1,		12,355			12,063			245			236
Service cost		267			243			1			1
Interest cost		305			409			8			10
Contributions by plan participants		76			81			—			—
Benefit payments		(614	)		(632	)		(15	)		(14	)
Benefit obligations of businesses acquired (divested)		—			(27	)		—			—
Actuarial (gain) loss		(469	)		1,536			(31	)		14
Plan amendments and other		(141	)		(64	)		(27	)		—
Exchange rate differences		(555	)		(1,254	)		(3	)		(2	)
Benefit obligation at December 31,		11,224			12,355			178			245
Fair value of plan assets at January 1,		10,465			10,930			—			—
Actual return on plan assets		(8	)		918			—			—
Contributions by employer		243			308			15			14
Contributions by plan participants		76			81			—			—
Benefit payments		(614	)		(632	)		(15	)		(14	)
Plan assets of businesses acquired (divested)		—			(25	)		—			—
Plan amendments and other		—			(68	)		—			—
Exchange rate differences		(419	)		(1,047	)		—			—
Fair value of plan assets at December 31,		9,743			10,465			—			—
Funded status—underfunded		(1,481	)		(1,890	)		(178	)		(245	)

Amount recognized in Accumulated other comprehensive loss

	December 31,
	2015			2014			2013			2015			2014			2013
($ in millions)	Defined pension benefits									Other postretirement benefits
Net actuarial loss		(2,383	)		(2,765	)		(2,050	)		(8	)		(39	)		(25	)
Prior service (cost) credit		127			2			(21	)		33			16			24
Amount recognized in OCI(1) and NCI(2)		(2,256	)		(2,763	)		(2,071	)		25			(23	)		(1	)
Taxes associated with amount recognized in OCI and NCI		512			652			459			—			—			—
Amount recognized in OCI and NCI, net of tax(3)		(1,744	)		(2,111	)		(1,612	)		25			(23	)		(1	)

(1)	OCI represent "Accumulated other comprehensive loss".
(2)	NCI represents "Noncontrolling interests".
(3)	NCI, net of tax, amounted to $0 million, $(3) million and $(3) million at December 31, 2015, 2014 and 2013, respectively.

Schedule of amounts recognized in balance sheet

	December 31,
	2015			2014			2015			2014
($ in millions)	Defined pension benefits						Other postretirement benefits
Overfunded plans		42			42			—			—
Underfunded plans—current		(18	)		(19	)		(14	)		(16	)
Underfunded plans—non-current		(1,505	)		(1,913	)		(164	)		(229	)
Funded status—underfunded		(1,481	)		(1,890	)		(178	)		(245	)

	December 31,
($ in millions)	2015		2014
Non-current assets
Overfunded pension plans		42		42
Other employee-related benefits		26		28
Prepaid pension and other employee benefits		68		70

	December 31,
($ in millions)	2015			2014
Current liabilities
Underfunded pension plans		(18	)		(19	)
Underfunded other postretirement benefit plans		(14	)		(16	)
Other employee-related benefits		(34	)		(40	)
Pension and other employee benefits (see Note 13)		(66	)		(75	)

	December 31,
($ in millions)	2015			2014
Non-current liabilities
Underfunded pension plans		(1,505	)		(1,913	)
Underfunded other postretirement benefit plans		(164	)		(229	)
Other employee-related benefits		(255	)		(252	)
Pension and other employee benefits		(1,924	)		(2,394	)

Difference of PBO and fair value of plan assets

	December 31,
	2015							2014
($ in millions)	PBO		Assets		Difference			PBO		Assets		Difference
PBO exceeds assets		10,413		8,890		(1,523	)		11,576		9,644		(1,932	)
Assets exceed PBO		811		853		42			779		821		42
Total		11,224		9,743		(1,481	)		12,355		10,465		(1,890	)

Difference of ABO and fair value of plan assets

	December 31,
	2015							2014
($ in millions)	ABO		Assets		Difference			ABO		Assets		Difference
ABO exceeds assets		8,781		7,496		(1,285	)		9,921		8,091		(1,830	)
Assets exceed ABO		2,143		2,247		104			1,948		2,374		426
Total		10,924		9,743		(1,181	)		11,869		10,465		(1,404	)

Component of net periodic benefit cost

	2015			2014			2013			2015			2014			2013
($ in millions)	Defined pension benefits									Other postretirement benefits
Service cost		267			243			249			1			1			1
Interest cost		305			409			373			8			10			9
Expected return on plan assets		(456	)		(481	)		(479	)		—			—			—
Amortization of prior service cost (credit)		38			27			34			(9	)		(9	)		(9	)
Amortization of net actuarial loss		127			99			136			1			—			4
Curtailments, settlements and special termination benefits		5			4			1			—			—			2
Net periodic benefit cost		286			301			314			1			2			7

Weighted-average assumptions, Benefit obligation

	December 31,
	2015		2014		2015		2014
(in %)	Defined pension benefits				Other postretirement benefits
Discount rate		2.63		2.61		3.63		3.49
Rate of compensation increase		1.53		1.65		—		—
Rate of pension increase		0.92		1.04		—		—

Weighted-average assumptions, Net periodic benefit cost

	2015		2014		2013		2015		2014		2013
(in %)	Defined pension benefits						Other postretirement benefits
Discount rate		2.61		3.58		3.22		3.49		4.17		3.35
Expected long-term rate of return on plan assets		4.58		4.60		4.79		—		—		—
Rate of compensation increase		1.65		1.81		1.71		—		—		—

Health care cost trend assumptions

	December 31,
	2015			2014
Health care cost trend rate assumed for next year		7.68	%		8.00	%
Rate to which the trend rate is assumed to decline (the ultimate trend rate)		5.00	%		5.00	%
Year that the rate reaches the ultimate trend rate		2028			2028

One-percentage-point change in assumed health care cost trend rates

        A one-percentage-point change in assumed health care cost trend rates would have the following effects at December 31, 2015:

	1-percentage-point
($ in millions)	Increase		Decrease
Effect on total of service and interest cost		—		(1	)
Effect on postretirement benefit obligation		13		(11	)

Target asset allocation on weighted-average basis

	Target percentage
Asset class
Equity		25
Fixed income		56
Real estate		11
Other		8
		100

Fair value of pension plan assets by asset category

	December 31, 2015
($ in millions)	Level 1		Level 2		Level 3		Total fair value
Asset class
Equity
Equity securities		364		—		—		364
Mutual funds/commingled funds		—		1,633		—		1,633
Emerging market mutual funds/commingled funds		—		328		—		328
Fixed income
Government and corporate securities		587		949		—		1,536
Government and corporate—mutual funds/commingled funds		—		3,257		—		3,257
Emerging market bonds—mutual funds/commingled funds		—		669		—		669
Real estate		—		74		1,106		1,180
Insurance contracts		—		121		—		121
Cash and short-term investments		160		219		—		379
Private equity		—		—		123		123
Hedge funds		—		—		94		94
Commodities		—		59		—		59
Total		1,111		7,309		1,323		9,743

	December 31, 2014
($ in millions)	Level 1		Level 2		Level 3		Total fair value
Asset class
Equity
Equity securities		433		—		—		433
Mutual funds/commingled funds		—		1,821		—		1,821
Emerging market mutual funds/commingled funds		—		487		—		487
Fixed income
Government and corporate securities		638		1,211		—		1,849
Government and corporate—mutual funds/commingled funds		—		3,521		—		3,521
Emerging market bonds—mutual funds/commingled funds		—		671		—		671
Real estate		—		94		842		936
Insurance contracts		—		126		—		126
Cash and short-term investments		274		56		—		330
Private equity		—		—		136		136
Hedge funds		—		—		93		93
Commodities		—		62		—		62
Total		1,345		8,049		1,071		10,465

Reconciliation of assets fair value using significant unobservable inputs (Level 3)

($ in millions)	Private equity			Hedge funds			Real estate			Commodities			Total Level 3
Balance at January 1, 2014		155			158			866			32			1,211
Return on plan assets
Assets still held at December 31, 2014		21			(3	)		43			(5	)		56
Assets sold during the year		3			8			—			—			11
Purchases (sales)		(39	)		(59	)		30			—			(68	)
Transfers from Level 3		—			—			—			(27	)		(27	)
Exchange rate differences		(4	)		(11	)		(97	)		—			(112	)
Balance at December 31, 2014		136			93			842			—			1,071
Return on plan assets
Assets still held at December 31, 2015		(9	)		1			54			—			46
Assets sold during the year		20			(1	)		(1	)		—			18
Purchases (sales)		(24	)		—			215			—			191
Exchange rate differences		—			1			(4	)		—			(3	)
Balance at December 31, 2015		123			94			1,106			—			1,323

Schedule of employer contributions to pension and other postretirement benefit plans

	2015		2014		2015		2014
($ in millions)	Defined pension benefits				Other postretirement benefits
Total contributions to defined benefit pension and other postretirement benefit plans		243		308		15		14
Of which, discretionary contributions to defined benefit pension plans		31		75		—		—

Expected future cash flows of pension and postretirement benefit plans

        The expected future cash flows to be paid by the Company's plans in respect of pension and other postretirement benefit plans at December 31, 2015, are as follows:

($ in millions)	Defined pension benefits		Other postretirement benefits
2016		641		15
2017		604		15
2018		604		14
2019		595		14
2020		583		14
Years 2021 - 2025		2,822		63